Vauxhall Vectra clicking sound from bottom end of engine

Sounds like an oil circulation fault . If not been done for a while , change oil and filter using good oil / filter . If it's coming from bottom of engine could be possibly small end but would really have to hear . You could also add an oil additive likes Wynns to help .

if its bottom end of engine it could be big end bearings,pistons or crankshaft,oil pump,best to get it checked by a garage as if left could cause major engine damage

If it was big end bearings , it wouldn't be a clicking noise and would get worse as engine warmed up . As you say it goes away when warm , ignore that suggestion .

for an accurate diagnosis take it to a garage where they can listen to the noise and find whats causing it